Frequency resources are relatively rare and expensive. The deployment of a network consists, inter alia, in calculating as well as possible the bandwidth necessary for each emitter. In operational mode, when an emitter or user does not need their resources, it is opportune that they can lend them to another one that does not have enough resources of its own to transmit all its data. The bitrate peaks of one emitter will thus be absorbed by the bitrate troughs of another.
The DVB-S2 standard has been developed for very-high-bitrate civil applications, for example for television. Once configured, a DVB-S2 carrier operates with a fixed symbol timing, that is to say a fixed bandwidth. The only possible adaptation known to date is to change frame-by-frame the modulation coding pair (MODCOD), this offering the advantage of resisting changes of propagation conditions having a dynamic swing of 15 dB. This adaptive coding & modulation mode is intrinsic to the standard but requires a return channel to inform the DVB-S2 emitter of the propagation conditions seen by the receiver. The MODCOD may differ from one frame to another, and therefore the spectral effectiveness also. In DVB-S2, a normal frame always transports 64800 bits, independently of the MODCOD used. It follows from this that a DVB-S2 frame does not have a number of fixed symbols and therefore a fixed duration. Table 1 gives for the so-called “normal” DVB-S2 frames, the number of symbols according to the modulation and the presence or otherwise of pilot symbols.
PilotlessWith PilotQPSK32490332828 PSK216902219416 APSK1629016686
The frames having different durations, it is not currently conceivable to temporally synchronize two DVB-S2 carriers. It is thus not possible to define a frequency plan which demands synchronization of the frames. FIG. 1 shows diagrammatically the distribution over time of the frequencies for several terminals. Thus, the user terminal 1 has a user band B1, the user terminal 2 has a frequency band B2. Each terminal has a pre-allocated band valid for the time of its mission.
The dynamic bitrate adaptation mechanism, known by the abbreviation DRA, may not be deployed in this case, it is therefore not possible for a DVB-S2 network to share its resources.
A solution for remedying this problem could be to constrain all the carriers to a single modulation/pilot pair, for example a pilotless phase shift keying modulation or QPSK (Quadrature phase-shift keying), but the benefit is greatly limited.
The current absence of synchronism in the known systems and methods also prevents the encryption of the DVB-S2 stream with implicit marker or Transec, the techniques of TRANSEC DVB-S2 requiring the transmission of the marker in clear, this being a flaw at the security level.
A need therefore exists to have a system and a method making it possible to obtain dynamic exchange of frequency resources in a system using the DVB-S2 standard, and in fact to have a frame with a suitably adapted structure.
In the subsequent description, the following expressions will be used:                “dummy” frame for mute frame, used when there is no information or data to be transmitted, and “dummy slots” for mute timeslots,        Super-frame ST, consisting of several DVB-S2 frames for a user, the frames optionally being separated by “dummy” frames, a super-frame constitutes the base entity for which all the DVB-S2 frames of a user Uk have the same MODCOD,        Jumbo-frame JT consisting of several super-frames, a jumbo-frame constitutes the temporal entity for which all the DVB-S2 frames of a user Uk have the same symbol bitrate,        the word “slot” defines a timeslot,        PSK corresponds to a modulation by change of phase or Phase-Shift Keying, QPSK with 4 PSK, APSK a modulation by change of amplitude or of phase or Amplitude Phase-Shift Keying.        
The expression “frame structure of fixed duration” corresponds to a frame structure which exhibits one and the same duration for several users.